This application describes a proposed examination of the hydroosmotic response of epithelia to the application of antidiuretic hormone (ADH). Investigations will be conducted on both amphibian urinary bladder and on isolated, perfused, cortical collecting tubules of the rabbit nephron. In each case the major mode of investigation will involve the use of differential interference-contrast microscopy (after Nomarski) with in vitro preparations in a manner that will allow simultaneous study of structure and function. The principal issue in question is the nature of cytoplasmic changes during ADH-mediated water flow so that we might understand better the physical basis for the documented discrepancy between hydraulic and diffusional permeabilities of hormone-stimulate tissue. In this regard, we plan to investigate the role of cytoskeletal elements (microtubules and microfilaments), structural changes in the luminal membrane, the dependent parameters for intercellular space geometry and the action of serosal hypertonicity. Direct visualization studies will be facilitated by closed-circuit video techniques and supplemented by periodic use of transmission electron microscopy.